In recent years, Internet of Things (IoT) networks are spreading. Often IoT connects devices such as sensors and gateways (GWs) using short-range wireless communication such as Bluetooth (registered trademark) and ZigBee (registered trademark) to construct a sensor network. However, sensor networks may suffer from failures due to external environmental changes or device malfunctions. When a failure occurs in the sensor network, it is difficult to collect user data (for example, temperature, humidity, and power generation amount) used by various IoT services. Therefore, it is important to collect data by stable operation of an IoT network.
For a solution, in the IoT network, in addition to the user data as mentioned above, operation management data contributing to the judgment on the communication status is sampled to detect a failure and to determine the content of the failure and the occurrence location. The operation management data is, for example, data indicating a device state for each device, including the wireless quality between devices, the usage status of a central processing unit (CPU) and a memory, and a battery remaining amount of each device. However, among such operation management data, data regarding the wireless quality varies exceptionally rapidly. Therefore, in order to constantly grasp the behavior of a wireless channel, the data regarding the wireless quality is sampled by each device at a short cycle at about several hundred millisecond intervals and transmitted to a GW. In particular, Bluetooth and ZigBee which are frequently used in the IoT network have an exceedingly narrow occupied bandwidth per channel as compared with a wireless local area network (LAN) or the like. Therefore, each device is desired to transmit data to the GW with high efficiency without any loss.
In order to cope with such a situation, a technique is known in which each device collectively transmits measurement data for a plurality of times with a maximum packet size allowed by a protocol. With this technique, high-efficiency data transmission with suppressed packet overhead is implemented.
Patent Document 1: Japanese Laid-open Patent Publication No. 2012-257016
However, when a plurality of devices transmits data with the maximum packet size, collision of packets is likely to occur between the devices, which increases the possibility of data loss. Therefore, even if the packet size is set to a maximum value, the data transmission efficiency does not necessarily increase. On the other hand, if data is transmitted with a smaller packet size, although the collision probability of the packets decreases, the overhead grows and the data transmission efficiency is lowered. Such a problem is particularly conspicuous in a narrowband IoT network.